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Electrostatics Forces and Fields The study of stationary charges and their interactions Electric Charge Two types of charge – “positive” and “negative”. Charge is measured in “coulombs” - C “e” – “elementary charge” – the amount of charge on a proton or electron e = 1.602 x 10-19 C (+ for proton;- for electron) “q” or “Q” is the variable used for charge Law of Charges - Law of Conservation of Charge – Opposite charges attract, like charges repel charge cannot be created or destroyed only transferred. So the net amount of charge produced in any process is zero. Conductors allow electrons to move freely, Insulators do not! Methods of Charging Charging by induction (conductors only) Charge distribution is changed due to influence of nearby charged object A grounding path is provided for charge movement Charging by contact (conductors and insulators) Actual contact by the charged object Polarization Polarization: a “separation of charge” is induced in an insulator by the influence of a charged object Electroscopes – a device to detect charge “Sharing” charge - charged objects will transfer charge to come to a better equilibrium If two charged conductors are connected (such as by a conducting wire) they will share charge to achieve a more balanced (shared) charge. -8mC 6mC -1mC -1mC Electric Forces – Coulomb’s Law +q1 + kq1q2 F 2 r F r + F +q2 Coulomb’s constant k = 9 x 109 Nm2/C2 Force interactions – “superposition” Linear – charges on an axis q1 Find Fnet on q3 q2 - 30 cm -8.0mC + +3.0mC q3 20 cm -4.0mC Fon3by1 = 1.2N Fnet = -F32 + F31 = -2.7N + 1.2 N = -1.5N Fon3by2 = 2.7N The Fnet is 1.5N and points to the left (Note: found magnitude only) Angles – use components F31 = 140N y Find Fnet on q3 F32 = 330N Find x and y components of each force acting on q3 – be careful of direction F32 Use Pythagorean theorem and tan-1 SEE NEXT SLIDE + q3 = 65mC F31 30cm 30o + q2 = 50mC 52cm - x q 3 = -86mC Equilibrium Two charges are on the x axis as shown. At what point must a third charge, q3 (2mc), be placed so the net force on it is zero? Set distance 3 to 2to “r” q1 15 mC Set distance from 1 to 3 as 2 -r r = 2.0 m F3,1 = F3,2 kq1q3 r2 = q2 6 mC Electric Fields ( N/C) Electric fields: vector quantity Electric fields: represented by “field lines” Definition: force per unit charge F E q Its action on a charge leads to a force on that charge F Eq A charge, Q, produces a field in space. This exerts a force (N) on a small “test” charge “q” placed in the field. F E q +Q q r Note: Q can be either + or – but the test charge (small q) is always + The E-field can also be calculated using the strength of the charge “Q” and the distance to a particular point in the field. kQ E 2 r E-Field lines 1. Field lines drawn away from positive and toward negative 2. Lines intersect surface at right angles 3. Number of lines is proportional to charge strength Interaction of field lines between charges of equal magnitude and opposite sign